Because of rapid development of mobile phones, etc. worldwide, mobile phones utilizing plural frequency bands and plural communications systems have been put into practical use. Because they should be operated as multiband terminals adapted for different frequency bands and communications systems, they have complicated circuits, resulting in increase in the number of parts and their size and cost. Accordingly, the reduction of the number of parts and the sharing of parts are actively pursued by circuit integration.
Because the sharing of a particularly large antenna greatly contributes to the miniaturization of terminals, it is important to develop a small antenna switch circuit capable of switching one antenna between plural bands.
Practically used in Europe as a universal mobile telecommunications system (UMTS) are triple-band mobile phones adapted for an extended global system for mobile communications (EGSM) of a 900 MHz band, a digital communication system (DCS) of a 1.8 GHz band, and a wideband code division multiple access (WCDMA) of a 2 GHz band.
For instance, EP1265370 (JP 2002-246942 A) discloses an antenna switch circuit capable of switching one antenna between three bands. In this antenna switch circuit, a first high-frequency switch comprises a pin-junction diode as a switching element to switch a GSM transmitting path and a GSM receiving path, and a second high-frequency switch comprises three sets of serially connected field effect transistors (FETs) as switching elements to switch a WCDMA transmitting/receiving path, a DCS transmitting path and a DCS receiving path.
These antenna switch circuits are generally supported by multilayer substrates. The multilayer substrate is obtained by printing an electrode material comprising silver or copper as a main component onto each ceramic sheet to form electrode patterns for transmission lines and capacitors for constituting the circuit, laminating these sheets, and sintering the resultant laminate. Switching elements such as pin-junction diodes, FET switches, etc. are usually mounted onto the upper surface of the multilayer substrate.
The circuit disclosed in EP1265370, however, fails to achieve an antenna switch applied to worldwide usable multiband mobile phones for four bands or more including not only EGSM and DCS, but also 850-MHz-band GSM850 (global system for mobile communications 850) and 1.9-GHz-band PCS (personal communication system) practically used in the U.S. This is because a GaAs-FET switch of SP6T as shown in FIG. 18 should be used when an antenna switch circuit adapted for four bands is constituted by using the GaAs-FET switch. Because the SP6T switch is generally larger and more expensive than conventional SPDT or SP3T, it is disadvantageous in the miniaturization and cost reduction of an antenna switch circuit. In addition, the control of the SP6T switch needs 6 or 7 control terminals, resulting in increase in the number of terminals in a module.
Multilayer substrates comprising FET switches are poor in isolation between transmitting paths and receiving paths. In the case of the GaAs-FET switch circuit shown in FIG. 18, isolation between transmission and receiving is only about 25 dB, needing improvement in a circuit structure for improved isolation characteristics. This results in a larger chip size of the GaAs-FET switch, and difficulty in miniaturization and cost increase. Because isolation characteristics between transmitting paths and receiving paths are affected by the interference of electrode patterns in the multilayer substrate, etc., the influence of the arrangement of electrodes, etc. in the multilayer substrate should be taken into consideration for the miniaturization of the module. However, EP1265370 fails to teach how the miniaturization and cost reduction of an antenna switch module, the improvement of isolation, electrode arrangement in the multilayer substrate, etc. should be achieved.